1. Field of the Invention
This invention relates, generally, to electrical machines. More specifically, it relates to a high efficiency electrical motor or generator.
2. Brief Description of the Prior Art
Electrical motors and generators are machines capable of converting electrical energy into mechanical energy and converting mechanical energy into electrical energy, respectively. These electrical machines have many similarities and can often be operated as either an electrical generator or motor.
Known electrical machines include a rotor, a stator, a plurality of electrical windings, and a plurality of permanent magnets. The rotor is a generally cylindrical shape with an outer circumference and an axial bore creating an inner circumference. The inner circumference is adapted to receive a shaft such that the two may rotate as one. The outer circumference of the rotor contains a plurality of permanent magnets (PMs) disposed longitudinally parallel to the axis of rotation or central axis of the rotor in a preferably generally uniform and concentric manner.
Current stator designs have a generally cylindrical shape with an outer circumference and a predetermined number of teeth projecting, a predetermined distance, inwardly towards the longitudinal axis of the stator. The teeth create a discontinuous inner circumference allowing the rotor and PM assembly to be disposed within the stator such that the rotor assembly is freely rotatable within the inner circumference of stator. Between each tooth is a stator slot having predetermined shape and size adapted to receive electrical windings. Electrical windings are typically strands of conductive materials, such as copper or aluminum, which are arranged into coil groups around the stator teeth. The electrical windings interact with the PMs to produce either mechanical or electrical energy. When configured as a motor, the electrical machine uses current flowing through the electrical windings to generate rotating magnetic fields which interact with the PMs attached to the rotor and cause the rotor and shaft to rotate. When configured as a generator, the PMs, and their respective magnetic fields, are rotated and interact with the electrical windings to produce electricity.
In recent years, the push towards green energy has increased the demand associated with developing efficient electric machine technology. The U.S. Department of Energy estimates that electric motors in the U.S. consume more than half of all electrical energy in the states. Therefore, improving the efficiency of these electric machines will greatly decrease the United States' carbon footprint.
Currently, some commercial off-the-shelf electric motors are designed for high efficiency, but have power densities on the order of only 0.1 horse power per pound (HP/lb.). Electric motors developed for use in aircraft propulsion (small unmanned aerial vehicles), on the other hand, have power densities between 1 and 2 HP/lb., while direct-drive electric motors can attain power densities greater than 5 HP/lb. only through increased operating speeds.
Traditionally in electric machine applications, the higher the electric machine's efficiency, the less energy wasted and the easier the thermal management system; however, the efficiency generally comes at the cost of increased size and weight. Copper loss is the term often given to heat produced by electrical currents in the conductors of transformer windings, or other electrical devices. Copper losses are an undesirable transfer of energy, as are core losses, which result from induced currents in adjacent components. Copper loss is the most significant in all the losses in electric machines, so reducing the copper loss is the key to building highly efficient machines. It is known that copper loss is inversely proportional to the wire's cross-sectional area. Therefore, copper wires having a greater cross-sectional area (large diameter) and lower current density will also require a larger slot area. The larger slot area requires the stator size to increase or tooth size to decrease. If the stator size increases, the machine becomes larger and the power density decreases. If the tooth size decreases, the magnetic saturation increases, and so the current must increase resulting in decreased efficiency.
Accordingly, what is needed is a highly efficient scalable permanent magnet machine having relatively high power density while being capable of operating at a wide range of power outputs. However, in view of the art considered as a whole at the time the present invention was made, it was not obvious to those of ordinary skill in the field of this invention how the shortcomings of the prior art could be overcome.
All referenced publications are incorporated herein by reference in their entirety. Furthermore, where a definition or use of a term in a reference, which is incorporated by reference herein, is inconsistent or contrary to the definition of that term provided herein, the definition of that term provided herein applies and the definition of that term in the reference does not apply.
While certain aspects of conventional technologies have been discussed to facilitate disclosure of the invention, Applicants in no way disclaim these technical aspects, and it is contemplated that the claimed invention may encompass one or more of the conventional technical aspects discussed herein.
The present invention may address one or more of the problems and deficiencies of the prior art discussed above. However, it is contemplated that the invention may prove useful in addressing other problems and deficiencies in a number of technical areas. Therefore, the claimed invention should not necessarily be construed as limited to addressing any of the particular problems or deficiencies discussed herein.
In this specification, where a document, act or item of knowledge is referred to or discussed, this reference or discussion is not an admission that the document, act or item of knowledge or any combination thereof was at the priority date, publicly available, known to the public, part of common general knowledge, or otherwise constitutes prior art under the applicable statutory provisions; or is known to be relevant to an attempt to solve any problem with which this specification is concerned.